Growth of cataclastic bands into a fault zone: A multiscalar process by microcrack coalescence in sandstones of Rio do Peixe Basin, NE Brazil

Abstract

We propose a structural evolutionary model for deformation band (DB) development via microcrack propagation and coalescence process in a normal fault zone affecting arkosic sandstones. Based on structural analysis and reconstruction of the paleostress, we infer that a sub-horizontal minimum principal stress (σ3) in the NNW-SSE direction controlled the multiscalar evolution of microcracks and DBs. Two main sets of coalesced microcracks and DBs are recognized: (1) NE-SW and (2) E-W, in which the coalescence pattern of microcracks is also observed among the DBs at meso-scale. Our results suggest that the DBs develop during three deformation stages: (1) pore collapse followed by initial grain breakage and the formation of NE-SW tensile microcracks favoured by cleavage planes in feldspars. These microcracks coalesce through linking NW-SE shear microcracks to form intermediary microplanes. An intense microcracking crush the grains, enhancing the cataclastic process. The further coalescence of these intermediary microplanes with other intermediary microplanes and/or microcracks might generate through-going microplanes inside the band, causing the formation of E-W DBs; (2) the E-W structures coalesce through NE-SW hard-linking DBs, which corresponds to the DBs clusters, generating NE-SW intermediary planes at meso-scale; (3) the NE-SW intermediary planes coalesce through an E-W hard-linking structure, which results in the DB fault zone.